This early build will apparently be for Xbox One owners only (Titanfall will also be released for the Xbox 360 and PC). So far, there's no word on how many people will be picked for the alpha test nor are there any gameplay details.

The full version of Titanfall, which will be released on March 11th, will be a multiplayer online only game as gamers take control of "pilots" on foot that can also get inside 25-foot tall mech-like machines called Titans. Respawn Entertainment will support up to 12 players in a match but will also throw in some elements like AI-controlled characters and chatter that are normally found in single player games.

Did you see the invite and delete it? If so, check your trash can and see if it's still available!